1. Field of the Invention
The present invention relates to an improved fuse cutout and, more particularly, to an improved fuse cutout which is less expensive to manufacture than prior art fuse cutouts, but which exhibits improved operating performance notwithstanding the decrease in expense thereof. The improved fuse cutout of the present invention may be used with a fuse link of the type described and claimed in commonly assigned, co-filed U.S. Pat. No. 4,317,099 issued Mar. 23, 1982 in the name of Richard J. Sabis. The cutout of this application may also include an improved fuse tube as described and claimed in commonly assigned, co-filed U.S. Pat. No. 4,313,100 issued Jan. 26, 1982 in the name of William E. Schmunk.
2. Discussion of the Prior Art
Fuse cutouts and fuse links therefor are well known. A typical fuse cutout includes a hollow insulative fuse tube having conductive ferrules mounted to the opposite ends thereof. One ferrule (often called the "exhaust" ferrule) is located at an exhaust end of the fuse tube and usually includes a trunnion casting which interfits with a trunnion pocket of a first contact assembly carried by one end of a porcelain or similar insulator. The other ferrule is normally held and latched by a second contact assembly carried by the other end of the porcelain insulator so that the fuse tube is normally parallel to, but spaced from, the porcelain insulator. The porcelain insulator is mountable to the cross-arm of a utility pole or a similar structure. The fuse link is located within the fuse tube with its ends respectively electrically continuous with the ferrules. One point of an electrical circuit is connected to the first contact assembly, while another point of the circuit is connected to the second contact assembly. Often, the insulator and the fuse tube are oriented perpendicular to the ground so that the exhaust ferrule and the first contact assembly are located below the other ferrule and the second contact assembly.
The fuse tube may include a high burst strength outer portion--for example, a fiber-glass-epoxy composite--lined with or containing an ablative arc-extinguishing material, such as horn fiber or bone fiber.
Normal currents flowing through the electrical circuit flow without affecting the fuse link. Should a fault current or other over-current, to which the fuse link is designed to respond, occur in the circuit, the fuse link operates as described below. Operation of the fuse link permits the upper ferrule to disengage itself from the upper contact assembly, whereupon the fuse tube rotates downwardly due to coaction of the trunnion casting and the trunnion pocket. If the fuse link operates properly, current in the circuit is interrupted and the rotation of the fuse tube gives a visual indication that the cutout has operated to protect the circuit.
Typical fuse links include a first terminal and a second terminal, between which there is normally connected a fusible element made of pure silver, silver-tin, or the like. Also connected between the terminals may be a strain wire, for a purpose described below. The second terminal is electrically continuous with, and is usually mechanically connected to, a button assembly, which is engagable by a portion of the upper ferrule on the fuse tube. The first terminal is connected to a flexible, stranded length of cable. Surrounding at least a portion of the second terminal, the fusible element, the strain wire (if used), the first terminal, and some portion of the flexible stranded cable is a sheath. The sheath is typically made of a so-called ablative arc-extinguishing material (such as horn fiber) or is a cellulosic material impregnated with an ablative arc-extinguishing material (such as boric acid). Such ablative arc-extinguishing materials are well known and comprise compounds or compositions which, when exposed to the heat of a high-voltage arc, ablate to rapidly evolve large quantities of deionizing turbulent and cooling gases. Typically, the sheath is much shorter than the fuse tube and terminates short of the exhaust end of the fuse tube.
The free end of the stranded cable exits the fuse tube from the exhaust end thereof and has tension or pulling force maintained thereon by a springloaded flipper on the trunnion casting. The tension or pulling force exerted on the cable by the flipper attempts to pull the cable and the first terminal out of the sheath and out of the fuse tube. The force of the flipper is normally restrained by the strain wire, typical fusible elements not having sufficient mechanical strength to resist this tension or pulling force.
In the operation of typical cutouts, a fault current or other over-current results, first, in the melting or vaporization of the fusible element, followed by the melting or vaporization of the strain wire. Following such melting or vaporization, a high-voltage arc is established between the first and second terminals within the sheath and the flipper is now free to pull the cable and the first terminal out of the sheath and, ultimately, out of the fuse tube. As the arc forms, the arc-extinguishing materials of the sheath begin to ablate and high quantities of de-ionizing, turbulent and cooling gases are evolved. The movement of the first terminal under the action of the flipper, and the subsequent rapid movement thereof due to the evolved gases acting thereon as on a piston, results in elongation of the arc. The presence of the de-ionizing, turbulent and cooling gas, plus arc elongation, may, depending on the level of the fault current or other over-current, ultimately result in extinction of the arc and interruption of the current at a subsequent current zero. The loss of the tension on the stranded cable originally effected by the flipper permits the trunnion casting to experience some initial movement relative to the exhaust ferrule which permits the upper ferrule to disengage itself from the upper contact assembly. This initiates a downward rotation of the fuse tube and its upper ferrule to a so-called "drop out" or "drop down" position.
As noted immediately above, arc elongation within the sheath and the action of the evolved gases may extinguish the arc. At very high fault current or over-current levels, however, arc elongation and the sheath may not, by themselves, be sufficient to achieve this end. Simply stated, at very high fault current levels, either the sheath may burst (because of the very high pressure of the evolved gas) or insufficient gas may be evolved therefrom to quench the high current level arc. For these reasons, the fuse tube is made of, or is lined with, ablative arc-extinguishing horn fiber or bone fiber, as noted above. In the event the sheath bursts, the arc-extinguishing material of the fuse tube interacts with the arc; gas evolved as a result thereof effects arc extinction. If the sheath does not burst, the arc-extinguishing material of the fuse tube between the end of the sheath and the exhaust end of the fuse tube is nevertheless available for evolving gas, in addition to that evolved from the sheath. The joint action of the two quantities of evolved gas, together with arc elongation, extinguish the arc.
Many manufacturers of cutouts and fuse links of the types described above continue to make concerted efforts to decrease the costs of the material and labor thereof, both as a matter of simple, good commercial practice, and in view of the fact that certain materials, such as silver, copper, and bronze, continue to experience large price increases. At the same time, manufacturers of cutouts and fuse links continue ongoing programs to improve the performance of these products. A less expensive, improved performance fuse cutout is a primary goal of the present invention.
The upper or second contact assembly usually includes a J-shaped spring contact, an end of the long leg of which is mounted for flexing to a rigid recoil bar. The recoil bar extends to a position between the legs of the J. A pin interconnects the legs of the J for conjoint movement. The pin passes through an aperture in a bushing pressed into, and extending beyond, a hole through the recoil bar and is connected to the opposite side of a concavity formed in the J's short leg. The concavity engages and holds an end of the fuse tube, as described below.
A spring acts between the recoil bar and the concavity to set a "rest" position of the J's short leg (and of the J's long leg). Upward deflection of the J's short leg is limited by abutment between the opposite side of the concavity and the bushing.
The lower or first contact assembly includes a feature--a projection, stud or the like--normally spaced from a similar feature or surface on the exhaust ferrule. The normal spacing between these features is substantially the same as the normal spacing between the opposite side of the concavity and the bushing.
When a fuse tube is properly positioned between the contact assemblies, the J's short leg is held away from its rest position against the bias of both the spring and the long leg to firmly engage a contact cap on the upper ferrule of the fuse tube in the concavity. When the fuse link operates, gases evolved within the fuse tube thrust it against the J's short leg in jet-like fashion further compressing the spring and flexing the J's long leg. The fuse tube may also randomly move the pin transversely at this time until the pin engages the walls of the aperture in the bushing. The random, transverse fuse tube movement may be viewed as precession of the pin between a pair of "pivots," one of which is the pin's connection to the J's short leg, the other of which is the envelope of the points of contact between the pin and the walls of the bushing aperture. This random, transverse fuse tube movement may have several deleterious effects.
First, ideally the features on the first contact assembly and the exhaust ferrule abut at the same time the opposite side of the concavity abuts the bushing. This transfers the forces on the fuse tube simultaneously to the contact assemblies. If the fuse tube moves too far transversely during its thrusting, these abutting events will not be simultaneous.
Second, ideally the contact cap should not disengage the concavity until the fusible elements of the fuse link completely melts to release the tension in the cable and until the initial thrust of the fuse tube subsides. Release of this tension and subsiding of fuse tube thrust permits a limited amount of relative movement between the exhaust ferrule and the trunnion casting about a toggle joint therebetween. This limited movement permits the contact cap to move out of the concavity and the fuse tube to fall "open" due to rotation of the trunnion casting in the trunnion pocket. If the fuse tube moves too far transversely during its thrusting, the contact cap may disengage the concavity too early.
Third, transverse movement of the fuse tube can apply a bending movement thereon. This bending movement can fracture the fuse tube near the exhaust ferrule.
Any solution to the above problems must take into account that presently available fuse tubes have "standard" lengths, which depend, inter alia, upon the voltage of the circuit to which the cutout is connected. Any such solution must not alter the cutout so as to require non-standard fuse tube lengths.
The present invention, then, is intended to solve the above-described problems, while achieving the overall goals of improving fuse cutout performance, while at the same time decreasing its cost.